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Herschel Breathes Its Last

The largest infrared space telescope ever launched has run out of cryogenic coolant, permanently ending its science operations.

Update: On June 17th mission controllers sent Herschel into its "disposal" orbit. The bon voyage marks the mission's official end. Read more in this ESA announcement.

The marvelous effects of massive star formation in the Carina Nebula appear in this Herschel image. Stellar winds and radiation have carved pillars and bubbles in the dense, dusty gas clouds. This image is a compilation of observations at 70, 160, and 250 microns.

The European Space Agency announced today that its infrared Herschel Space Observatory has finally kicked the light bucket.

Herschel launched May 14, 2009 with the Planck satellite and was the first space observatory to cover far-infrared to submillimeter wavelengths, its observations spanning 55 to 672 microns. Both spacecraft went to L2, the second Lagrange point in the Sun-Earth gravitational system where a small mass can basically “hover” without being pulled this way or that.

Herschel’s 3.5-meter primary mirror worked with two cameras (both with spectroscopic abilities) and a high-res spectrometer, all of which were cryogenically cooled by superfluid liquid helium to a couple of degrees above absolute zero, -271°C (-456°F) — as least until Herschel ran out of coolant.

Mission planners estimated that Herschel would exhaust its helium supply in late March. It lasted one month beyond that projection. But without coolant its science operations are now impossible.

The familiar visible-light view of the Horsehead nebula (left) compared to a far-infrared internal view from the Herschel Space Observatory, which reveals regions of intense star formation.

Steve Eales / Univ. of Cardiff

Herschel’s main objectives were to study the formation of stars and early galaxies, the universe’s molecular chemistry, and the chemical composition of solar system atmospheres and surfaces. Observing in infrared is key for this work: infrared wavelengths reveal stuff a few tens of degrees above absolute zero (such as nebulae) and also pass through the dusty clouds enshrouding young stars. In addition, cosmic expansion has shifted visible light from the early universe to infrared wavelengths, meaning studies of that era must be conducted in infrared. (Incidentally, that’s why the infamous James Webb will focus on infrared, too — the infrared range is possibly the most information-heavy of the entire electromagnetic spectrum.)

For far-infrared observers, Herschel was a major upgrade from NASA’s Spitzer Space Telescope, which has a primary mirror only 0.85 meters wide and observed from 3 to 180 microns while it had coolant. (Spitzer is still working in “warm mode” and observes at 3.6 and 4.5 microns — i.e. it only sees the toastier stuff now.)

Herschel allowed astronomers to zoom in on the formation of massive stars, those that will someday cataclysmically die to form neutron stars and black holes. Its observations revealed that these stars grow on the margins of rapidly expanding bubbles in the interstellar medium, where the expanding bubbles squeeze surrounding gas. In the distant universe, Herschel detected evidence that galaxies' star formation spiked within the first few billion years, and that it's the availability of gas, not the fireworks of galaxies crashing into each other, that fuels such bursts.

ESA's Herschel Space Observatory obtained this view of cold gas and dust in the Andromeda Galaxy at four far-infrared wavelengths. The blue channel shows emission at 70 microns, green at 100 microns, and red at 160 and 250 microns combined. The image spans roughly 3°. Click here for a larger view.

ESA / PACS & SPIRE Consotrium / O. Krause / H. Linz

It also revealed that the water ice of Comet 103P/Hartley 2 has the same hydrogen-deuterium ratio as Earth’s water — a much-sought data point for those who argue Earth’s water comes from comets. However, Hartley 2 is the only comet with the right ratio, and other data suggest that the comet is actually not such a great match to Earth’s composition after all. Increasingly, astronomers are arguing that Earth formed with its water and didn’t need alien delivery.

Another water discovery came in late April from Herschel: the detection of water in Jupiter's upper atmosphere, left there by Comet Shoemaker-Levy 9 in 1994. Even though it's been nearly 20 years since the comet smashed into Jupiter, Herschel's observations show more water in the southern hemisphere (where the comet hit) than in the north and suggest that 95% of the planet's current water comes from that comet.

Using Herschel, astronomers found out that what they’d long thought was a dark cloud hiding star formation next to the reflection nebula NGC 1999 is actually a hole in space. While black in visible light, the purported cloud should have shown up in infrared — and it wasn’t there. Instead, nearby stars somehow blew out that section of gas.

With its usefulness ended, Herschel will have to leave L2. The mission team will send the spacecraft into a “no-return heliocentric orbit,” which means it will take at least 300 years to return to the Earth-Moon system’s gravitational playground. Maybe by then, space retrieval systems will be so good that astronauts will grab it and bring it back for 24th-century children to gawk at in the Smithsonian.

About Camille M. Carlisle

Science Editor Camille M. Carlisle covers science news for Sky & Telescope. She specializes in black holes, galactic cosmology, and whatever she happens to be writing about at the time. She and Robert Naeye co-edited S&T's special issue Mars: Mysteries and Marvels of the Red Planet, published summer 2014.

3 thoughts on “Herschel Breathes Its Last”

I find that Ms. Camille Carlyle’s commentaries on current (especially beyond solar system) astronomical discoveries , both observational and theoretical, to be heads above anything else on the Internet. Please pass on my comments to the editor of Sky and Telescope, with whom I went on the trip to to Chile in the fall of 2012. I t is a rare talent to be able to communicate the substance as well as the excitement of the scientific process that leads to the discoveries.

A very fine article, Camille. I would like to understand a point: you mention that Herschel’s science operations are now impossible. While it is clear that no further infrared observations can be carried out, according to the science objectives it was specifically designed for, I am wondering if any of its detectors can still operate under ambient temperatures…a telescope specifically designed to detect infrared is still a telescope which can continue to observe, and its position above the Earth’s atmosphere might yet yield some imaging results that may provide a fair degree of scientific interest. My question is whether the shut down of this wonderful spacecraft after its coolant has run out is due to actual physical shortcomings associated with its detectors which make any further observations physically impossible or because of a shortcoming in funding that prevents an otherwise (potentially) serviceable space telescope from continuing to provide optical-wavelength observations?

To Starfart: Nope, the end of observations are due to the temperature, not funding. Herschel wasn’t designed to work at higher temperatures, so mission planners knew they couldn’t use it once the coolant was gone. You can read more about Herschel’s instruments at http://sci.esa.int/jump.cfm?oid=34691. To Nick: Thanks. My colleague forwarded your kind post to the whole office!

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